1. Field of the Invention
This invention relates to a magneto-optical recording medium, and in particular to a magneto-optical recording medium from which signals are reproduced by domain wall displacement detection (DWDD).
2. Description of the Related Art
Due to the transition to digital information of recent years, it has become possible to record large quantities of digital data, and there has arisen strong market demand for recording media with inexpensive unit cost.
As optical recording media technology enabling large capacities, a magneto-optical recording medium employing domain wall displacement detection (DWDD) for reproduction of magneto-optical recording signals has such features as a resolution which is not in principle limited by the limiting optical resolution of the reproduction light, determined by the optical system wavelength and the numerical aperture (NA) of the objective lens; the ability to obtain large reproduction signals from minute recorded domains; and the ability to achieve extremely high linear recording densities.
In the basic configuration of this DWDD medium, the magnetic layer has a three-layer structure comprising, from the side of incidence of reproduction light, a magnetic layer constituting a reproduction layer, a magnetic layer constituting a blocking layer or a so-called switching layer, and a magnetic layer constituting a recording layer. At room temperature, these magnetic layers are in a state in which the recording layer and reproduction layer are magnetically exchange-coupled through the blocking layer, and recorded domains in the recording layer are transferred to the reproduction layer; by causing the magnetic properties of the blocking layer to disappear through a rise in temperature caused by irradiation of reproduction light, the magnetic coupling between the reproduction layer and recording layer is blocked, and this higher temperature distribution causes an expansion of recorded domains in the reproduction layer within the spot of reproduction light. Consequently excellent reproduction characteristics can be obtained even from minute recorded domains, that is, at higher recording densities.
However, for reasons that are explained in detail below, there is the problem that due to the principle of detection employed in this DWDD medium, ghost signals occur in addition to the main signals.
In response to this there has been a proposal of a magneto-optical recording medium and a signal reproduction method to suppress the occurrence of ghost signals (International Patent Publication No. WO99/39342, from the 10th line of page 5 to the 6th line of page 13, FIGS. 1 to 18).
This magneto-optical recording medium adopts a four-layer structure which adds a control layer (a layer to suppress domain wall displacement) between the reproduction layer (displacement layer) and the blocking layer (switching layer) of the above-described magnetic layer with a three-layer structure.
In this case, the blocking layer and control layer have the respective Curie temperatures TC2 and TC3, wherein TC2>TC3.
In this medium, the control layer (suppression layer) suppresses displacement of domain walls from behind the reproduction light spot, which is the origin of the occurrence of ghost signals, so as to suppress the occurrence of ghost signals.
According to the invention disclosed in the above-described International Patent, although the occurrence of ghost signals is alleviated, domain wall displacement is suppressed, so that there is the possibility of a decline in reproduction output, that is, of the occurrence of dropout; reproduction is particularly easily affected in land and groove recording at high linear densities. Hence in order to improve recording and reproduction characteristics using the invention disclosed in the above International Patent, ghost signals must be effectively suppressed at a desired linear density, and moreover the configuration of the magnetic layer must be designed carefully such that dropout does not occur.
The present invention was arrived at through the discovery of a magneto-optical recording medium with simplified magnetic layer design and a signal reproduction method, which avoid the occurrence of ghost signals and enable improvement of dropout.